A LAN is a digital communications network interconnecting several local computer workstations through a shared medium. It is also possible to connect two or more LANs. The communication network interconnecting remote LANs is referred to generally as a WAN (wide area network). In one form, a WAN is a network containing some switching mechanism to which each LAN is connected by a bridge. In another form, a WAN is a simple point-to-point link for interconnection of only two remote LANs.
U.S. Pat. No. 4,901,312, issued Feb. 13, 1990 to Hui et al describes remote interconnection of LANs by a virtual or pseudo ring formed in a WAN by e.g., a packet switching network.
U.S. Pat. No. 5,329,527, issued Jul. 12, 1994 to Ujihashi et al describes a system in which two or more LANs are interconnected by way of an ATM network by respective bridges to effect communication between terminals. In the system of the patent, each bridge has an address table for local terminals and a second table containing logical connection identifiers to other bridges and addresses of remote terminals connected to other bridges.
An ATM service called "LAN emulation" that emulates services of existing LANs across an ATM network allows the use of the vast base of existing LAN application software. By using such a service, end systems (e.g. workstations, servers, bridges, etc.) can connect to the ATM network while the software applications interact as if they are attached to a traditional LAN.
There are several advantages that a LAN emulation based on ATM (or other switching technologies) has over a true shared medium LAN. A LAN emulation:
breaks geographical limitations on the separation of stations participating on the same LAN segment, in particular when a station is moved to a new location within the same enterprise it could remain in the same LAN segment; PA1 can accommodate many more stations on the same LAN segment; and PA1 can be partitioned into LAN segments based upon functional requirements, not geographical proximity.
The ATM Forum is defining a technology of emulating shared media LAN segments on ATM networks. It regards the LAN emulation as important for both accommodating existing LAN-based protocols on ATM stations and for providing connectivity between ATM stations and stations on "legacy" LANs. The architecture underpinning the ATM Forum's effort is based upon servers. For example, a server provides address resolution, i.e. obtaining the ATM address representing the station with a particular address, and a multicast server handles multicast traffic. A station can also open a point-to-point VCC (virtual channel connection) to another station once it has found its ATM address. Besides a direct data transfer through a VCC, the source can also make use of a data forwarder server. A server station sends packets to the data forwarder server when it does not know the destination's ATM address. The data forwarder server re-transmits the packets to the alternate destination.
Separate VCCs are needed to each of these servers, and each requires its own packet format. Thus, an attempt to send a short packet to a hitherto unused address can result in ten or more ATM level transactions. Also, when a station swaps between using the data forwarder service and a point-to-point connection, there is a risk that its packets will arrive out of order. Since mis-ordering does not occur in shared media LAN segments, the ATM Forum is proposing an extra protocol to flush one path before the other is used.
Servers are notoriously difficult to engineer; they have a high start-up cost for small systems and they don't scale well. The present invention simplifies greatly a LAN emulation over an ATM network without the use of servers and will help wide acceptance of ATM networks to integrate existing LANs.